This invention relates to hot runner valves, and more particularly, to hot runner valves designed for cooling the motive mechanism thereof during the operation of the valve.
Hot runner systems can be categorized into two types with respect to the method of closing off the mold cavity injection gate. These types include a thermally closed gate and a mechanically closed gate. This invention relates to mechanical or valve gate closing mechanisms for use in multi-cavity or high cavitation molds and molding systems as well as single cavity molds. Typically, a valve gated actuating mechanism is a unitized device which is attached to a valve stem or other commonly known gate closing component. Accordingly, valve stem actuation devices typically consume a considerable amount of space within a mold platen. As a result of such large space consumption, molds are formed which are too large for typical injection molding machine, resulting in increased expense due to the necessity to use larger and more materials for producing a larger mold to accommodate the mechanism.
Such a scenario typically arises when a valve gated hot runner system is desired for a multi-level or stack mold. Most valve gated actuation mechanisms assume a large space which would be better used for an opposing injection nozzle housing arrangement. Such is the case for typical mechanically actuated closed gate mechanisms and a typical thermally activated closed gate mechanisms. In comparing two such mechanisms, it is obvious that the mechanically closed gate is generally significantly larger than the thermally closed gate. Accordingly, it would be beneficial in the art to design a mechanically actuated closed gate system of a size comparable to thermally closed gate systems.
German patent 1,133,880 shows a nozzle suitable for attachment to the end of an injection molding machine extruder. The actuating mechanism used to move the valve stem in reciprocating fashion is shown as an annular piston, where a pressurized fluid is employed as the motive force. As with all piston type actuators, it is necessary to provide resilient seals which serve to prevent pressure leak from the pressurized chambers on both sides of piston, so that maximum force is transferred to the piston. Additionally, pressurized fluid leakage can lead to, wasting energy or fluid substance; creating undesirable noise; fire hazards; and undesirable cooling effects on the hot melt conveying components adjacent to it. The resilient seals for this nozzle design must be of a very high temperature capability.
Plastic conveying equipment, such as that described in the German patent, often needs to operate at temperatures well over 500.degree. F. Resilient seals which can survive in such an environment for desirable periods of time are either unavailable or require that a more complex, multiple-piece piston design be used. Additionally, such seals are prohibitively expensive and will not provide a 100% seal over extended periods of time. Although the German patent does not show or describe the method of piston sealing, it is presumed that it suffers from the sealing/leaking problems as discussed above.
U.S. Pat. No. 4,082,226 shows another gate valve actuating mechanism. An annular piston of complex and bulky design is used, which includes many parts requiring high manufacturing expense and laborious assembly time. By necessity, the piston seals must withstand a very high temperature to provide prolonged service next to the hot medial portion of the nozzle. The very bulky piston design, because of its ratio of height to diameter, is prone to cocking and jamming should one of the piston posts 50 show resistance to slide due to sticking or friction. Also, as seen in FIG. 7 of the patent, the valve stem 66 must hit the outlet bore 71 to stop travel of the stem. Such contact can lead to undesirable wear and possibly the damage of the bore and front nozzle portion.
U.S. Pat. No. 4,443,178 shows a compact method of actuating a valve stem using a spring, as shown in FIG. 8. However, this method relies on plastic pressure to push the valve stem back and the spring pressure is not readily adjustable with respect to force or time desired of the stem to close the gate. A pressurized piston is far superior in its ability to readily vary stem force as well as permit actuation of the stem while pressure still exists in the system or delay closing of the stem even after pressure has been released.
U.S. Pat. No. 4,832,593 shows a design similar to the aforementioned patent, but where the motive means, in this case an air piston, is not annular in shape. The piston is solid and is positioned on the center axis and directly behind the nozzle housing. Because the piston resides within the heated body used to convey plastic melt to the nozzle housing, it requires a cast iron piston ring as a sealing device to withstand the high temperatures. Such metal-on-metal dynamic seals inherently do not provide 100% sealing efficiency and thereby are not capable of allowing maximum supply pressure to act on the piston face. Also, it can be seen from FIG. 1 that the nozzle body is necessarily much larger in diameter than the nozzle itself and the axial length of the nozzle and nozzle body together is extended, due to the internal space required to provide the piston assembly.
All of the above cited patents are not adaptable for use inside an injection mold frame, especially in a mold where molding cavity spacing is dense, so as to maximize production output from the molding tool. Nor do they permit the design of a multi-level or stack mold with a minimum mold open distance, compatible with standard injection machines. Also, the prior art does not disclose an appropriate piston assembly design or piston seal which overcomes leaking, wear or attrition in a very hot environment.
There exists, therefore, a need in the injection molding art for a mechanically actuated valve gated system which is self-cooling and space efficient.